1. Field of Invention
The present invention relates to a piezoelectric device, such as a piezoelectric resonator, piezoelectric oscillator and SAW device used for electronic appliances. In particular, the invention relates to a package structure for a piezoelectric device, wherein a lid made of a thin glass sheet is bonded to a base that mounts a resonator element made of piezoelectric material, such as quartz crystal, by using low-melting point glass, and the resonator element is then hermetically sealed.
2. Description of Related Art
In accordance with the related art, various types of information and communication equipment and electronic appliances, such as OA machines and household electronic appliances, extensively use piezoelectric devices as a clock source for electronic circuits, such as a piezoelectric resonator, an oscillator including a piezoelectric resonator element and an IC chip sealed in the same package, and a real time clock module. In particular, a piezoelectric device must be miniaturized, flattened and suitable for surface mounting on a circuit board in response to the downsizing of the overall device, such as compact information appliances, including for example mobile computers and IC cards, and communication appliances, including for example cellular phones.
In general, a surface-mounted piezoelectric device has a package structure in which a lid is bonded to a base that mounts a piezoelectric resonator element and, if necessary, an IC device to be sealed. In particular, a related art technique uses a transparent glass material for the lid so that laser beams can be irradiated onto the piezoelectric resonator element from the exterior of the device to adjust the frequency after the sealing. FIGS. 9(A) and (B) show an example of a related art piezoelectric resonator having this package structure. The piezoelectric resonator has a package 3 including a base 1 made of insulating material and a lid 2, in which a tuning fork type quartz crystal resonator element 4 is hermetically sealed. The base 1 is a rectangular box made of laminated thin sheets of insulating material, such as ceramics, on the bottom of which the quartz crystal resonator element 4 is fixed at its base portion 4a in a cantilever manner, with conductive adhesive. The rectangular thin sheet lid 2 is hermetically bonded at its lower surface to the top surface of the base 1 by using low-melting point glass 5.
A package for a piezoelectric device is subject to, for instance, shock when dropped and external force when gripped by jigs during the mounting operation. Therefore, sealing strength is required for ensuring a sufficient hermetic state in the bonded and sealed part between the base and lid. However, the related art package structure described above simply bonds the top surface of a base to the lower surface of a lid using low-melting point glass. The horizontal joint surface receives the entire force, particularly, from the sides of the package. Therefore, to prevent the hermetic state at the bonded and sealed part from being impaired, which may result in performance loss or defects of the piezoelectric device, the lid is conventionally bonded to the top surface of the base with a predetermined sealing width (w1 in FIG. 9(A)), which is then surrounded by low-melting point glass 5 having a sufficient width (w2 in FIG. 9(A)), in order to enhance the strength of the device against lateral external force.
On the other hand, the base 1 is produced generally by laminating and bonding a predetermined number of large ceramic thin sheets 6 having a desired shape and size, and cutting it along the dotted lines 7 in FIG. 10 into a number of bases having a predetermined length and width. Here, castrations (round through holes) 8 are made at the intersections of the horizontal and vertical dotted lines 7 to facilitate cutting. This leaves a quarter circle shaped cutout on the corners of the divided base 1. When the rectangular lid is laid on it, as shown in FIG. 9(C), the corner of the base 1 does not have enough margin to hold low-melting point glass 5 outside the lid 2, or the corner of the lid 2 may even protrude from the outer limit of the base. In addition, if low-melting point glass 5 runs down the outer wall of the base 1, as shown in FIG. 9(C), a lateral external force may cause the low-melting point glass to crack, possibly impairing the hermetic state of the package. Conversely, if a margin is provided for the low-melting point glass to travel beyond the sealing width w1, sufficient bonding and sealing strength is ensured. However, this makes the package larger in length and width, limiting miniaturization of a piezoelectric device.
If less low-melting point glass is used to prevent the glass from running down the outer wall of the base 1, the low-melting point glass may not reach the periphery of the lid 2, and a space 10 may be defined between the lid 2 and the base 1, as shown in FIG. 9(D). If the package with the lid 2 and base 1 bonded in this way is subject to an external force from above, the space 10 may allow the initiation of cracking or breaking of the lid 2 that is made of glass material, and the hermetic state of the package may also be impaired.
The present invention addresses the problems in the related art described above, and an object of the invention is to provide a package structure for a piezoelectric device, that includes a package having a base on which a piezoelectric resonator element is mounted, and a lid that is made of glass material and bonded to the top surface of the base using low-melting point glass, the package structure ensuring sufficient bonding and sealing strength and enabling miniaturization and flattening of the device.
To attain the above object, the present invention provides a package for a piezoelectric device, including a base having a rectangular box shape, and a lid that is made of thin rectangular glass sheet and bonded onto the top surface of the base using low-melting point glass. A piezoelectric resonator element is hermetically sealed. The corners of the rectangular lid are cut off so that a margin with a predetermined width is defined entirely between the outer periphery of the base and the periphery of the lid.
Bonding the lid that is formed in this way to the base ensures a sufficient sealing width on the top surface of the base around the lid and a sufficient margin width for low-melting point glass to travel around. This provides sufficient bonding and sealing strength against a lateral external force applied to the package.
The corners of the lid can be formed differently than described above. The corners can be diagonally cut off in one embodiment, and rounded in another embodiment, for example. The lid can easily be processed into a desired shape using machining, chemical processes, such as etching, or monolithic molding, such as embossing.
In an embodiment, the cut off corners of the rectangular lid are tapered to the upper surfaces. This provides each corner of the package, which is highly possibly affected by a direct external force in the course of handling the piezoelectric device, with a sufficient sealing width and a margin at the corners of the lid described above. In addition, this tapered surface reduces horizontal force on each joint surface between the base top surface and the lid at each corner because it transforms the lateral force on the package into horizontal and vertical elements. In this way, a minor additional process to the lid ensures a practically sufficient bonding and sealing strength. In another embodiment, the lid is tapered to the upper surface along the periphery, providing enhanced bonding and sealing strength to the package.
In another embodiment, the lid is stepped on the upper surface thereof around the periphery. This reduces the possibility for the jigs to be trapped while handling the piezoelectric device for assembly, preferably reducing the risk of undesired external force to the package. The step can easily be formed by machining, chemical processes, such as etching, or monolithic molding, such as embossing.
In another embodiment, the lid is stepped on the lower surface thereof that engages with the inner periphery of the base. Bonding the step along the inner periphery of the base facilitates and ensures their positioning. Furthermore, the package can be reduced in height by the height of the step, realizing a flattened piezoelectric device. The step can easily be formed in a desired form by attaching low-melting point glass onto the lower surface of the lid in addition to the methods described above, i.e., machining, chemical processes, such as etching, or monolithic molding, such as embossing.
In another embodiment, the base has a through hole by which the inside of the package communicates with the exterior of the device and the through hole is hermetically closed with a sealing medium. The package can be vacuum sealed by filling the through hole in a vacuum atmosphere with a sealing medium after bonding the base to the lid. When the lid is bonded to the base using low-melting point glass, the heat may cause the gas to expand or occur in the package. In that case, particularly with a smaller package, increased pressure inside of the package may change the sealing width of low-melting point glass and affect the hermetic state.
However, in this embodiment, the gas inside of the package can be released via the through hole of the base to prevent the inside pressure from increasing. This maintains and ensures the hermetic state of the package. Furthermore, if a transparent lid is used, a laser beam can be irradiated from outside the lid after the package is sealed to evaporate and remove metal weight material that was previously attached to the surface of the piezoelectric resonator element, thereby enabling frequency adjustment.
Another aspect of the present invention provides a piezoelectric device including a package that is vacuum sealed as described above, and a tuning fork type piezoelectric resonator element having a base portion and a pair of resonating arms on the surfaces of which excitation electrodes are formed and sealed in the package. The tuning fork type piezoelectric resonator element has excitation electrodes including first electrodes that are formed on the main surfaces of the resonating arms, and second electrodes that are formed on the side surfaces of the resonating arms. The first electrodes include an electrode film that is coated on the side surfaces of a groove formed lengthwise at least on one of the main surfaces of the resonating arms.
The tuning fork type piezoelectric resonator element having the structure above is known, as is described for instance in Japanese Laid-Open Patent S56-65517. It is known that electric field efficiency can be significantly increased by producing an electric field parallel to the main surfaces of the resonating arms, which also suppresses the CI (Crystal Impedance) value.
However, the existence of gas, such as air, or an insufficient vacuum within the package, causes extra air resistance against the grooves formed on the main surfaces of the resonating arms during the flexural vibration of the resonating arms. A smaller tuning fork type piezoelectric resonator element, which results from downsizing the piezoelectric device, is subject to more restriction on the flexural vibration of the resonating arms and, therefore, the CI value may not be as low as expected. The piezoelectric device according to the present invention suppresses gas production from the low-melting point glass and conductive adhesive. The package can be sealed, with the inside being subject to a high vacuum. This ensures sufficient flexural vibration of the resonating arms and a sufficiently low CI value.
The present invention further provides a piezoelectric device including a package that can be vacuum sealed, and a tuning fork type piezoelectric resonator element having a base portion that is fixed on the base of the package and a pair of resonating arms on the surfaces of which excitation electrodes are formed and sealed in the package. The base portion of the tuning fork type piezoelectric resonator element has a constriction formed between the part fixed to the base and the resonating arms.
The tuning fork type piezoelectric resonator element blocks the resonation of the resonating arms at the constriction and does not allow it to reach the fixed part of the base portion. Therefore, the resonating arms have more freedom of flexural vibration and the CI value is maintained lower. However, the flexural vibration of the resonating arms is more restricted when the existence of gas, such as air, or insufficient vacuum within the package causes extra air resistance against the constriction. A smaller tuning fork type piezoelectric resonator element, which results from the downsizing of the piezoelectric device, is subject to more restriction on the flexural vibration of the resonating arms and, therefore, the CI value may not be as low as expected. The piezoelectric device according to the present invention also minimizes gas production from low-melting point glass or conductive adhesive. The package can be sealed, with the inside being subject to a high vacuum. This ensures more freedom of flexural vibration for the resonating arms as well as a sufficiently low CI value.
Another aspect of the present invention provides a piezoelectric device in which an IC device is further mounted in the package described above.